By “NOx specific reactant” herein, we mean a reducing agent that, in most conditions, preferentially reduces NOx instead of other components of a gaseous mixture. Examples of NOx-specific reactants include nitrogenous compounds such as nitrogen hydrides, e.g. ammonia (NH3) or hydrazine, or an NH3 precursor.
By “NH3 precursor” we mean one or more compounds from which NH3 can be derived, e.g. by hydrolysis. These include urea (CO(NH2)2) as an aqueous solution or as a solid or ammonium carbamate (NH2COONH4). If the urea is used as an aqueous solution, a eutectic mixture, e.g. a 32.5% NH3 (aq), is preferred. Additives can be included in the aqueous solutions to reduce the crystallisation temperature.
Urea hydrolyses at temperatures above 160° C. according to equation (1) to liberate NH3 itself. It also thermally decomposes at this temperature and above according to equations (2) and (3) resulting in reduction of NOx.CO(NH2)2+H2O→2NH3+CO2  (1)CO(NH2)2→.NH2+CO  (2).NH2+NO→N2+H2O  (3)
The NH3 can be in anhydrous form or as an aqueous solution, for example.
The application of NH3 SCR technology to treat NOx emissions from IC engines, particularly lean-burn IC engines, is well known. Several chemical reactions occur in the NH3 SCR system, all of which represent desirable reactions which reduce NOx to elemental nitrogen. The dominant reaction mechanism is represented in equation (4).4NO+4NH3+O2→4N2+6H2O  (4)
Competing, non-selective reactions with oxygen can produce secondary emissions or may unproductively consume NH3. One such non-selective reaction is the complete oxidation of NH3, represented in equation (5).4NH3+5O2→4NO+6H2O  (5)
Presently, urea is the preferred source of NH3 for mobile applications because it is less toxic than NH3, it is easy to transport and handle, is inexpensive and commonly available.
Early methods of using urea as a source of NH3 in exhaust systems involved injecting urea directly into the exhaust gas, optionally over an in-line hydrolysis catalyst (see EP-A-0487886 (incorporated herein by reference)). However, not all urea is hydrolysed in such arrangements, particularly at lower temperatures.
Incomplete hydrolysis of urea can lead to increased PM emissions on tests for meeting the relevant emission test cycle because partially hydrolysed urea solids or droplets will be trapped by the filter paper used in the legislative test for PM and counted as PM mass. Furthermore, the release of certain products of incomplete urea hydrolysis, such as cyanuric acid, is environmentally undesirable. Another method is to use a pre-injection hydrolysis reactor (see U.S. Pat. No. 5,968,464 (incorporated herein by reference)) held at a temperature above that at which urea hydrolyses.
It will be appreciated that at lower temperatures, below about 100–200° C., NH3 can also react with NO2 to produce explosive ammonium nitrate (NH4NO3) according to equation (6):2NH3+2NO2+H2O→NH4NO3+NH4NO2  (6)
For the avoidance of doubt, the present invention does not embrace such reactions or the promotion of conditions which bring them about. For example, the reaction can be avoided by ensuring that the temperature does not fall below about 200° C. or by supplying into a gas stream less than the precise amount of NH3 necessary for the stoichiometric reaction with NOx (1 to 1 mole ratio). For cold start applications, measures to prevent water from contacting the catalyst can be adopted. These can include disposing a water trap, e.g. a zeolite, upstream of the catalyst to reduce the amount of water vapour contacting the catalyst until it is heated sufficiently. A water trap can also be positioned downstream of the catalyst, to prevent atmospheric humid air from travelling up the exhaust pipe. An electric heater can also be employed to drive off moisture from the catalyst pre-cold start. Such arrangements are described in our EP 0747581, (incorporated herein by reference).
In order to meet existing and future emission legislation, generally a vehicular exhaust system includes one or more components, such as catalysts. One of the legislated exhaust gas components is NOx. During normal operation, the exhaust gas produced by a lean-burn internal combustion engine, for example, includes an excess of oxygen and oxidising species. It is very difficult to reduce NOx to N2 in an oxidising or lean atmosphere. In order to treat NOx in lean exhaust gases, a component has been developed that absorbs NOx during normal lean-burn operation of the engine. This component is commonly called a NOx-trap and generally it includes: (i) an oxidation catalyst (e.g. platinum) to oxidise NO in the exhaust gas to NO2 in the oxidising atmosphere; (ii) a NOx storage component to store the NO2 e.g. as the nitrate. The NOx storage component is generally a basic compound of an alkali metal or an alkaline-earth, such as barium oxide; and (iii) a reduction catalyst, such as rhodium. It is possible, however, to use a NOx trap formulation in certain circumstances which comprises only the NOx storage component, or the NOx storage component and one or other of the oxidation and reduction catalyst.
Intermittently, the engine is run rich, e.g. by adjusting the moment of fuel injection into one or more cylinders, or by injecting a reducing agent, e.g. a hydrocarbon fuel, into the exhaust gas, in order to remove the stored NOx and reduce it to N2. This also regenerates the absorber for another store-regenerate cycle.
Another approach for removing NOx from a gas stream is by selective catalytic reduction (SCR), which comprises adding e.g. NH3 to the gas and passing the mixture over a catalyst effective to react the NOx and NH3 to nitrogen. Another approach is described in our WO 00/21647 (incorporated herein by reference) wherein NOx from a diesel engine exhaust gas is removed by absorbing it in a solid absorbent. The absorbent is regenerated by the action of a NOx-specific reactant.
Either such process requires careful control to avoid over- or under-supply of e.g. NH3, leading respectively to emission of NH3 or NOx. NH3 is an irritant and has an unpleasant odour and, accordingly, it is undesirable to slip NH3 to atmosphere. In practice this would mean positioning an oxidation “clean-up” catalyst downstream of the SCR catalyst or NOx-trap to oxidise slipped NH3 to NOx. Therefore, slipping NOx per se, by providing inadequate levels of NH3, or NH3 would have the effect of reducing the overall effectiveness of the exhaust system to limit emissions.
One problem associated with NH3 SCR technology is to maintain good NOx conversion when the exhaust gas temperature is relatively low, e.g. during extended idling or following cold-start. NOx conversion can be achieved using NH3 at temperatures as low as 150° C. using Pt-based catalysts, but the preferred delivery form of NH3, aqueous urea solution, does not decompose significantly below 200° C. Pt-based catalysts generate nitrous oxide (NO) according to equation (5) at above about 225° C.
Changes in engine test cycles have been introduced in the present Euro III standard and will govern Euro IV type approval of new vehicles. In particular, the new test cycles include the European Stationary Cycle (ESC); the European Transient Cycle (ETC); and a test for smoke opacity on the European Load Response (ELR) test. These cycles and tests include significant periods at low temperature. To attain type approval, a new vehicle will have to pass both the ETC and ESC/ELR tests.
It is stated in our WO 00/21647 that “If the [NH3 SCR] catalyst system is associated with the [NOx] absorbent, that is the absorber is ‘catalysed’, the catalytic material may be for example co-precipitated or co-impregnated or co-deposited with NOx absorbent or present as one or more sandwiched layers or as fine (e.g. 10–500 microns) particles on or in a layer of absorbent or among particles of absorbent”.
Elsewhere in WO 00/21647 we state that the point of injection of a NOx specific reactant can be downstream of the filter, i.e. upstream of the NOx absorbent and “in this event the temperature is typically in the range 150–300° C.”.
We have looked at putting barium (a NOx absorbent) on a cerium and iron-containing SCR catalyst, and the SCR function was significantly reduced, although the ability of the composition to absorb and desorb NOx was unimpaired. In our WO 02/068099 we demonstrate the principle of using NH3 (or urea) injection over a NOx trap catalyst to reduce stored NOx during lean running conditions. This arrangement is particularly useful for diesel applications. In WO 02/068099 we state: “Whichever [NOx storage] compounds are used, there may be present also one or more catalytic agents, such as precious metals, effective to promote reactions of NOx-specific reactant [including NH3] with nitroxy salt. Such catalysts are also known as SCR catalysts and can include iron/zeolite or V2O5/TiO2. Where the NOx absorbent and SCR catalyst are associated, in one embodiment they are segregated. By ‘segregated’ we mean that they should, at least, be supported on separate supports and can therefore be disposed in separate layers above and/or below the other component or in the same layer. Alternatively, they can be coated on distinct areas of the same substrate ‘brick’ or on separate substrates disposed within the same system.”
We have now found that it is possible to use a NOx absorbent disposed downstream of a SCR catalyst to prevent NH3 slip from an SCR catalyst and NOx and NH3 slip from the system as a whole. We have also found that it is possible to regenerate a NOx absorbent in a manner set out in our WO 02/068099 by intentionally slipping a controlled amount of NH3 past an SCR catalyst when the catalyst is above a pre-determined temperature.